Solid electrolytic capacitor and method for manufacturing same

ABSTRACT

A solid electrolytic capacitor comprises an anode formed of at least one metal selected from tantalum, niobium, titanium and tungsten, and a dielectric layer, an electrolytic layer and a cathode disposed on the anode, wherein the cathode comprises a mixed layer containing a first material consisting of silver particles having an average particle diameter (median diameter) of not less than 2 μm, a second material consisting of conducting carbon particles and/or silver particles having an average particle diameter (median diameter) of 1 μm or less and a binding agent.

TECHNICAL FIELD

The present invention relates to a solid electrolytic capacitor and afabrication method therefor. More particularly, the invention relates toa solid electrolytic capacitor comprising an anode formed of at leastone metal selected from tantalum, niobium, titanium and tungsten, adielectric layer formed on the anode, and an electrolytic layer and acathode disposed on the dielectric layer, characterized in that thecathode is improved and an equivalent series resistance (ESR) in highfrequency regions is decreased.

BACKGROUND ART

Solid electrolytic capacitors are conventionally used for personalcomputers and so on.

Recently, an instant supply of current to a circuit has been requiredbecause of increase in frequency of CPUs for use in personal computers.Therefore, development of a solid electrolytic capacitor featuring adecreased equivalent series resistance in high frequency regions hasbeen demanded.

As such solid electrolytic capacitors, solid electrolytic capacitorsincluding an anode formed of a metal, such as tantalum and so on, adielectric layer of an oxide of such a metal formed on the anode, and anelectrolytic layer and a cathode deposited on the dielectric layer aregenerally used.

As the cathode of such solid electrolytic capacitors, two layers of acarbon layer and a silver layer deposited on the electrolytic layer aregenerally used.

However, the cathode of the two layers of the carbon layer and thesilver layer has a problem as follows. Because a contact resistancebetween the carbon layer and the silver layer is increased due todifferences in properties between the carbon layer and the silver layer,the equivalent series resistance in the high frequency regions isincreased.

Therefore, for example, JP-A-10-242000 discloses a solid electrolyticcapacitor including the cathode comprising three layers of a carbonlayer, a mixed layer of carbon and silver and a silver layer formed onan electrolytic layer for the purpose of decreasing the equivalentseries resistance in the high frequency regions.

However, according to results of examinations made by inventors of thepresent invention, in the case of forming the three layers of the carbonlayer, the mixed layer of carbon and silver, and the silver layer as thecathode on the electrolytic layer as mentioned above, the contactresistance between the mixed layer of carbon and silver and the silverlayer was increased and it was still impossible to sufficiently decreasethe equivalent series resistance in the high frequency regions.

DISCLOSURE OF THE INVENTION

The present invention is made in order to solve the aforementionedproblems. Also, it is an object of the invention to decrease a contactresistance and so on to obtain a solid electrolyte capacitor with adecreased equivalent series resistance in high frequency regions.

According to the invention, a solid electrolytic capacitor comprises ananode formed of at least one metal selected from tantalum, niobium,titanium and tungsten and a dielectric layer, an electrolytic layer anda cathode disposed on the anode, wherein the cathode comprises a mixedlayer containing a first material consisting of silver particles havingan average particle diameter (median diameter) of not less than 2 μm, asecond material consisting of conducting carbon particles and/or silverparticles having an average particle diameter (median diameter) of 1 μmor less and a binding agent.

As the aforementioned solid electrolytic capacitor, in a case where themixed layer containing the first material consisting of the silverparticles having the median diameter of not less than 2 μm, the secondmaterial consisting of the conducting carbon particles and/or the silverparticles having the median diameter of 1 μm or less and the bidingagent is formed as the cathode on the electrolytic layer, the particlesof the second material consisting of the conducting carbon particlesand/or the silver particles having the median diameter of 1 μm or lessenter spaces between the particles of the first material consisting ofthe silver particles having the median diameter of not less than 2 μm,so that conductivity in the mixed layer is improved and adheringproperty between the mixed layer and the electrolytic layer is enhanceddecreasing the contact resistance thereof. As a result, the equivalentseries resistance in the high frequency regions is notably decreased.

In the solid electrolytic capacitor, as the cathode, a carbon layer maybe formed in addition to the mixed layer. Such a carbon layer is formedbetween the electrolytic layer and the mixed layer.

In a case where the carbon layer is formed on the electrolytic layer asmentioned above and the mixed layer containing the first materialconsisting of the silver particles having the median diameter of notless than 2 μm, the second material consisting of the conducting carbonparticles and/or the silver particles having the median diameter of 1 μmor less and the biding agent is formed on the carbon layer, theparticles of the second material consisting of the conducting carbonparticles and/or the silver particles having the median diameter of 1 μmor less enter the spaces between the particles of the first materialconsisting of the silver particles having the median diameter of notless than 2 μm in the mixed layer, so that the conductivity in the mixedlayer is improved and the adhering property between the mixed layer andthe carbon layer is enhanced decreasing the contact resistance. As aresult, the equivalent series resistance in high frequency regions isnotably decreased.

Further, in the mixed layer of the solid electrolytic capacitor, when anamount of the second material consisting of the conducting carbonparticles and/or the silver particles having the median diameter of 1 μmor less is insufficient, the particles of the second material consistingof the conducting carbon particles and/or the silver particles havingthe median diameter of 1 μm or less do not sufficiently enter the spacesbetween the particles of the first material consisting of the silverparticles having the median diameter of not less than 2 μm, so that itbecomes difficult to obtain the above mentioned results. On the otherhand, when the amount of the second material is excessive, the amount ofthe first material having a larger particle diameter becomesinsufficient. As a result, electricity moves passing through a lot ofcontact areas, and the contact resistance between particles is increasedcausing increase in the equivalent series resistance. In view of theabove, the amount of the second material based on a total amount of thefirst material and the second material is preferably set in a range of0.5 to 40 wt %, and more preferably in a range of 3 to 40 wt %.

In the solid electrolytic capacitor, it is difficult to obtain thesilver particles having the median diameter of 1 μm or less to be usedin the mixed layer by grinding. Therefore, it is preferable to reducesilver oxide particles having the median diameter of 1 μm or less toobtain silver particles having the median diameter of 1 μm or less.

In order to obtain the mixed layer containing the silver particleshaving the median diameter of 1 μm or less, the silver oxide particleshaving the median diameter of 1 μm or less are contained in the mixedlayer and then the silver oxide particles are reduced.

In a case where the silver oxide particles having the median diameter of1 μm or less are contained in the mixed layer and reduced, as a methodof reduction, for example, heat-treatment at not less than 160° C. ispreferable.

Further, in the solid electrolytic capacitor, when the contact areabetween the silver particles of the first material and the silverparticles of the second material is enlarged, the conductivity in themixed layer is enhanced and the equivalent series resistance isdecreased. In order to obtain such a result, it is preferable to usescale-shaped silver particles of which ratio of a thickness to a lengthis very small as the silver particles of the first material and thesecond material.

Usable scale-shaped silver particles as the first material are thescale-shaped silver particles having a median in a maximum length of notless than 2 μm and the usable scale-shaped silver particles as thesecond material are the scale-shaped silver particles having the medianin the maximum length of 1 μm or less. The maximum length in the silverparticles is a length of a line drawn maximumly between two points onthe periphery of the particles. The median in the maximum length is alength of a cumulated point on the cumulated distribution curve of themaximum length of these particles becoming 50%.

When a ratio of the maximum length L to thickness d (L/d) in each of thescale-shaped silver particles is too small, it becomes difficult tofurther decrease the equivalent series resistance by enlargement of thecontact areas between the silver particles of the first material andsilver particles of the second material. On the other hand, when theaforementioned ratio (L/d) is too large, an amount of the binding agentcovering surfaces of the silver particles becomes excessive, so that theequivalent series resistance is increased. In view of the above, it ispreferable to use scale-shaped silver particles having the ratio of themaximum length L to the thickness d (L/d) set in a range of 4 to 100.

As the same as the aforementioned solid electrolytic capacitor, in bothcases where the scale-shaped silver particles having the median in themaximum length of not less than 2 μm are used as the first material andthe scale-shaped silver particles having the median in the maximumlength of 1 μm or less are used as the second material, when the amountof the second material in the mixed layer is insufficient, the particlesof the second material do not sufficiently enter the spaces between theparticles of the first material, so that the aforementioned results cannot be obtained. On the other hand, when the amount of the secondmaterial is excessive, the amount of the first material becomesinsufficient and electricity moves passing through a lot of contactareas, resulting in increase in the contact resistance and theequivalent series resistance in high frequency regions. In view of theabove, the amount of the second material based on the total amount ofthe first material and the second material is preferably set in a rangeof 0.5 to 40 wt %, more preferably in a range of 3 to 40 wt %.

Further, as the same as the silver particles having the median diameterof 1 μm or less, it is difficult to obtain the scale-shaped silverparticles having the median in the maximum length of 1 μm or less bygrinding. Therefore, it is preferable to reduce the silver oxideparticles having the median in the maximum length of 1 μm or less toobtain the silver particles having the median in the maximum length of 1μm or less.

Further, in order to obtain the mixed layer containing the scale-shapedsilver particles having the median in the maximum length of 1 μm orless, the silver oxide particles having the median in the maximum lengthof 1 μm or less are contained in the mixed layer and then the silveroxide particles are reduced.

In a case where the scale-shaped silver oxide particles having themedian in the maximum length of 1 μm or less are contained in the mixedlayer and reduced, as the method for the reduction, for example, theheat-treatment at not less than 160° C. is preferable.

As the conducting carbon particles in the second material, for example,carbon black or graphite can be used. In particular, it is preferable touse a mixture of carbon black and graphite.

As the binding agent for the mixed layer, it is possible to usewell-known binding agents which has conventionally used. In particular,it is preferable to use at least one resin selected from polyimideresin, epoxy resin and polyester resin. In the case of using at leastone resin selected from polyimide resin, epoxy resin and polyester resinas the binding agent, the adhesive characteristics between the carbonlayer and the mixed layer is further enhanced and the equivalent seriesresistance in the high frequency regions is further decreased in a firstsolid electrolytic capacitor wherein the carbon layer is formed, and theadhesive characteristics between the electrolytic layer and the mixedlayer is further enhanced and the equivalent series resistance in highfrequency regions is further decreased in a second solid electrolyticcapacitor wherein the carbon layer is not formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a solid electrolytic capacitoraccording to Embodiment 1 of the invention;

FIG. 2 is a sectional view illustrating a solid electrolytic capacitoraccording to Embodiment 2 of the invention;

FIG. 3 is a sectional view illustrating a solid electrolytic capacitorfabricated in Comparative Example 1; and

FIG. 4 is a sectional view illustrating a solid electrolytic capacitorfabricated in Comparative Example 2.

BEST MODES FOR CARRYING OUT THE INVENTION

A solid electrolytic capacitor according to Embodiments of the inventionwill be described hereinbelow in details with reference to theaccompanying drawings.

Embodiment 1

As shown in FIG. 1, a solid electrolytic capacitor according toEmbodiment 1 has the following structure. An anode 1 formed of at leastone metal selected from tantalum, niobium, titanium and tungsten isanodized to form a dielectric layer 2 of an oxide on a surface thereof,an electrolytic layer 3 comprising such as conducting polymer andmanganese dioxide is formed on the dielectric layer 2, and a cathode 4is formed on the electrolytic layer 3.

In the solid electrolytic capacitor of Embodiment 1, a carbon layer 4 ais formed as the cathode 4 on the electrolytic layer 3 and a mixed layer4 b containing a first material consisting of silver particles having amedian diameter of not less than 2 μm, a second material consisting ofconducting carbon particles and/or silver particles having a mediandiameter of 1 μm or less and a binding agent is formed on the carbonlayer 4 a.

It is possible to use scale-shaped silver particles having the median inthe maximum length of not less than 2 μm as the first material insteadof the silver particles having the median diameter of not less than 2μm. Also, it is possible to use the scale-shaped silver particles havingthe median in the maximum length of 1 μm or less as the second materialinstead of the silver particles having the median diameter of 1 μm orless.

Embodiment 2

As shown in FIG. 2, a solid electrolytic capacitor according toEmbodiment 2 has the following structure. The anode 1 formed of at leastone metal selected from tantalum, niobium, titanium and tungsten isanodized to form the dielectric layer 2 of an oxide on the surfacethereof, the electrolytic layer 3 formed of such as conducting polymerand manganese dioxide is formed on the dielectric layer 2, and thecathode 4 is formed on the electrolytic layer 3.

In the solid electrolytic capacitor of the Embodiment 2, the carbonlayer 4 a is not formed as the cathode 4 on the electrolytic layer 3,only the mixed layer 4 b containing the first material consisting of thesilver particles having the median diameter of not less than 2 μm, thesecond material consisting of the conducting carbon particles and/or thesilver particles having the median diameter of 1 μm or less and thebinding agent is formed.

In the solid electrolytic capacitor of Embodiment 2, it is possible touse scale-shaped silver particles having a median in a maximum length ofnot less than 2 μm as the first material instead of the silver particleshaving the median diameter of not less than 2 μm. Also, it is possibleto use the scale-shaped silver particles having the median in themaximum length of 1 μm or less as the second material instead of thesilver particles having the median diameter of 1 μm or less.

EXAMPLES

Hereinafter, a solid electrolytic capacitor according to examples of theinvention will specifically be described while comparative examples willbe cited to demonstrate that equivalent series resistance in highfrequency regions in the inventive solid electrolytic capacitor ofexamples is notably decreased. It is to be noted that the solidelectrolytic capacitor of the invention should not be limited to thefollowing examples thereof and suitable changes and modifications may bemade thereto within the scope of the invention.

Example A1

A solid electrolytic capacitor of Example A1 has the same structure ofthe solid electrolytic capacitor of Embodiment 1 as described above.

In the solid electrolytic capacitor according to Example A1, an anodeformed of tantalum sintered body was anodized to form a dielectric layer2 of an oxide film on a surface thereof, an electrolytic layer 3consisting of polypyrrole of a conducting polymer obtained byelectrolytic polymerization and so on was formed on the dielectric layer2, and a cathode 4 was formed on the electrolytic layer 3.

In forming the cathode 4 on the electrolytic layer 3, a carbon pasteprepared by mixing graphite (5 wt %), water (90 wt %) and carboxymethylcellulose as a binding agent (5 wt %) was coated on the electrolyticlayer 3 and dried at a temperature of 150° C. for 30 minutes to form acarbon layer 4 a on the electrolytic layer 3.

On the other hand, a paste for the mixed layer was prepared by admixing88 parts by weight of a mixture wherein silver particles of nearlyspherical form having a median diameter of 3 μm and silver oxide (I)Ag₂O particles of nearly spherical form having a median diameter of 0.5μm were mixed in a weight ratio of 95:5 with 5 parts by weight ofpolyamideimide which is one of polyimide resin as the binding agent and7 parts by weight of Y-butyrolactone as a solvent.

Next, the paste for the mixed layer was coated on the carbon layer 4 aand dried at 160° C. for 30 minutes reducing the silver oxide (I) Ag₂Oparticles to form the mixed layer 4 b containing the silver particleshaving the median diameter of 3 μm and the silver particles having themedian diameter of 0.5 μm bound by polyamideimide.

Example A2

In a solid electrolytic capacitor of Example A2, silver oxide (II) AgOparticles of nearly spherical form having a median diameter of 0.5 μmwas used instead of the silver oxide (I) Ag₂O particles of nearlyspherical form having the median diameter of 0.5 μm for preparing thepaste for the mixed layer of the solid electrolytic capacitor of ExampleA1. Except for the above, the same procedure as in Example 1 was used toform the mixed layer 4 b containing the silver particles having themedian diameter of 3 μm and the silver particles having the mediandiameter of 0.5 μm bound by polyamideimide.

Example A3

In a solid electrolytic capacitor of Example A3, acetylene black whichis one of carbon black having a median diameter of 0.05 μm was usedinstead of the Ag₂O particles having the median diameter of 0.5 μm forpreparing the paste for the mixed layer of the solid electrolyticcapacitor of Example A1. Except for the above, the same procedure as inExample A1 was used to form the mixed layer 4 b containing the silverparticles having the median diameter of 3 μm and acetylene black havingthe median diameter of 0.05 μm bound by polyamideimide.

Example A4

In a solid electrolytic capacitor of Example A4, a mixture wherein thesilver particles of nearly spherical form having the median diameter of3 μm, the Ag₂O particles of nearly spherical form having the mediandiameter of 0.5 μm and acetylene black having the median diameter of0.05 μm were mixed in a weight ratio of 95:2.5:2.5 was used forpreparing the paste for the mixed layer of the solid electrolyticcapacitor of Example A1. Except for the above, the same procedure as inExample A1 was used to form the mixed layer 4 b containing the silverparticles having the median diameter of 3 μm, the silver particleshaving the median diameter of 0.5 μm and acetylene black having themedian diameter of 0.05 μm bound by polyamideimide.

Example B1

A solid electrolytic capacitor of Example B1 has the same structure ofthe solid electrolytic capacitor of Embodiment 2 as described above.

In a solid electrolytic capacitor of Example B1, in forming the cathode4 of the solid electrolytic capacitor of Example A1, the carbon layer 4a was not formed on the electrolytic layer 3 consisting of polypyrrole.Except for the above, the same procedure as in Example A1 was used toform the cathode 4 consisting of the mixed layer 4 b containing thesilver particles having the median diameter of 3 μm and the silverparticles having the median diameter of 0.5 μm bound by polyamideimideon the electrolytic layer 3.

Comparative Example 1

In a solid electrolytic capacitor of Comparative Example 1, a silverpaste prepared by admixing 88 parts by weight of the silver particles ofnearly spherical form having the median diameter of 3 μm with 5 parts byweight of polyamideimide and 7 parts by weight of Y-butyrolactone as thesolvent was used instead of the paste for the mixed layer used in thesolid electrolytic capacitor of Example A1. Except for the above, thesame procedure as in Example A1 was used to form a silver layer 4 cwherein the silver particles having the median diameter of 3 μm werebound by polyamideimide as shown in FIG. 3.

Comparative Example 2

In a solid electrolytic capacitor of Comparative Example 2, the sameprocedure as in Example A3 was used to form the mixed layer 4 bcontaining the silver particles of nearly spherical form having themedian diameter of 3 μm and acetylene black having the median diameterof 0.05 μm bound by polyamideimide. Then, as shown in FIG. 4, the silverpaste prepared by admixing 88 parts by weight of the silver particleshaving the median diameter of 3 μm with 5 parts by weight ofpolyamideimide and 7 parts by weight of Y-butyrolactone as the solventwas coated on the mixed layer 4 b and dried at 150° C. for 30 minutes toform the silver layer 4 c on the mixed layer 4 b.

The resultant solid electrolytic capacitors of Examples A1 to A4,Example B1 and Comparative Examples 1 and 2 were each determined for theequivalent series resistance (ESR) by means of an equivalent seriesresistance meter at a frequency of 100 kHz. An index number of theequivalent series resistance (ESR) of each of the solid electrolyticcapacitors was determined on a basis of the equivalent series resistance(ESR) of the solid electrolytic capacitor of Example A1 defined as 100.The results are listed in Table 1 as below. TABLE 11 COMPARATIVE EXAMPLEEXAMPLE A1 A2 A3 A4 B1 1 2 ESR 100 100 102 97 102 160 150

As is apparent from the table, each of the solid electrolytic capacitorsof Examples A1 to A4 wherein the respective carbon layer 4 a and themixed layer 4 b containing the first material consisting of the silverparticles having the median diameter of not less than 2 μm and thesecond material consisting of the conducting carbon particles and/orsilver particles having the median diameter of 1 μm or less were formedon the electrolytic layer 3 and the solid electrolytic capacitor ofExample B1 wherein the carbon layer 4 a was not formed and the mixedlayer 4 b was formed directly on the electrolytic layer 3 presented anotably decreased ESR as compared with the solid electrolytic capacitorof Comparative Example 1 wherein the silver layer 4 c containing thesilver particles having the median diameter of 3 μm was formed insteadof the mixed layer 4 b and the solid electrolytic capacitor ofComparative Example 2 wherein the silver layer 4 c containing only thesilver particles having the median diameter of 3 μm was formed on themixed layer 4 b.

According to a comparison among the solid electrolytic capacitors ofExamples A1 to A4 and Example B1, the solid electrolytic capacitorsusing the second material containing the silver particles having themedian diameter of 1 μm or less presented a decreased ESR as comparedwith the solid electrolytic capacitor of Example A3 using the secondmaterial containing only acetylene black of the conducting carbonparticles. In particular, the ESR in the solid electrolytic capacitor ofA4 using the second material containing both the silver particles havingthe median diameter of 1 μm or less and acetylene black of theconducting carbon particles was further decreased.

Examples A5 and A6

The same procedure as in Example A3 was used to fabricate each of solidelectrolytic capacitors of Example A5 and A6, except that a type of theconducting carbon particles contained in the mixed layer 4 b waschanged.

As the conducting carbon particles, graphite having the median diameterof 5 μm was used in Example A5 and a mixture prepared by mixingacetylene black having the median diameter of 0.05 μm and graphitehaving the median diameter of 5 μm in a weight ratio of 1:1 was used inExample A6.

In the same manner as above, the resultant solid electrolytic capacitorsof Examples A5 and A6 were each determined for the equivalent seriesresistance (ESR) at the frequency of 100 kHz. Then, an index number ofthe equivalent series resistance (ESR) of each of the solid electrolyticcapacitors of Examples A5 and A6 was determined on the basis of theequivalent series resistance (ESR) of the solid electrolytic capacitorof Example A3 defined as 100. The results are listed in Table 2 asbelow. TABLE 2 TYPE OF CONDUCTING CARBON PARTICLES ESR EXAMPLE A3acetylene black 100 EXAMPLE A5 graphite 105 EXAMPLE A6 acetylene black +graphite 97

As is apparent from the table, as the solid electrolytic capacitor ofExample A3 using acetylene black as the conducting carbon particlescontained in the mixed layer 4b, each of the solid electrolyticcapacitors of Examples A5 and A6 using graphite or the mixture ofacetylene black and graphite presented a notably decreased ESR ascompared with the solid electrolytic capacitors of Comparative Examples1 and 2.

Further, the solid electrolytic capacitors wherein acetylene blackhaving a small particle diameter was contained as the conducting carbonparticles in the mixed layer 4 b presented a decreased ESR as comparedwith the solid electrolytic capacitor of Example A5 wherein onlygraphite having a large particle diameter was contained as theconducting carbon materials in the mixed layer 4 b. In particular, theESR in the solid electrolytic capacitor of Example A6 using the mixtureof acetylene black having the small particle diameter and graphitehaving the large particle diameter was further decreased.

Examples A7 and A8 and Comparative Examples 3 and 4

The same procedure as in Example A1 was used to fabricate each of solidelectrolytic capacitors of Examples A7 and A8 and Comparative Examples 3and 4, except that the median diameter of the Ag₂O particles to be addedto the paste for the mixed layer was changed and the median diameter ofthe silver particles having a small particle diameter to be mixed to thesilver particles having the median diameter of 3 μm in the mixed layer 4b was changed.

Example A7 used Ag₂O particles having a median diameter of 0.1 μm,Example A8 used Ag₂O particles having a median diameter of 1.0 μm,Comparative Example 3 used Ag₂O particles having a median diameter of1.5 μm and Comparative Example 4 used Ag₂O particles having a mediandiameter of 2.0 μm. When the Ag₂O particles were dried at 160° C. for 30minutes and reduced into silver particles in the same manner as above,the silver particles having the same median diameter as before reductionwere contained in the mixed layer.

In the same manner as above, the resultant solid electrolytic capacitorsof Examples A7 and A8 and Comparative Examples 3 and 4 were eachdetermined for the equivalent series resistance (ESR) at the frequencyof 100 kHz. Then, an index number of the equivalent series resistance(ESR) of each of the solid electrolytic capacitors of Examples A7 and A8and Comparative Examples 3 and 4 was determined on the basis of theequivalent series resistance (ESR) of the solid electrolytic capacitorof Example A1 defined as 100. The results are listed in Table 3 asbelow. TABLE 3 AVERAGE PARTICLE DIAMETER OF SILVER OXIDE(I) Ag₂O (μm)ESR EXAMPLE A7 0.1 98 EXAMPLE A1 0.5 100 EXAMPLE A8 1 110 COMPARATIVEEXAMPLE 3 1.5 150 COMPARATIVE EXAMPLE 4 2 160

As is apparent from the table, in each of the solid electrolyticcapacitors of Examples A1, A7 and A8 using silver particles having themedian diameter of 1 μm or less as the silver particles having the smallparticle diameter to be mixed with the silver particles having themedian diameter of 3 μm in the mixed layer 4 b, the ESR was notablydecreased as compared with the solid electrolytic capacitors ofComparative Examples 3 and 4 using silver particles having the mediandiameter of more than 1 μm as the silver particles having the smallparticle diameter to be mixed with the silver particles having themedian diameter of 3 μm.

Examples A9 to A18

In Examples A9 to A18, the same procedure as in Example A4 was used toform the mixed layer 4 b containing the mixture of the silver particleshaving the median diameter of 3 μm, the Ag₂O particles having the mediandiameter of 0.5 μm and acetylene black having the median diameter of0.05 μm, except the weight ratio thereof was changed.

Each weight ratio was set in 99.75:0.125:0.125 in Example A9,99.5:0.25:0.25 in Example A10, 99:0.5:0.5 in Example A11, 98:1:1 inExample A12, 97:1:2 in Example A13, 90:5:5 in Example A14, 80:15:5 inExample A15, 60:35:5 in Example A16, 55:40:5 in Example A17 and 50:45:5in Example A18. Except for the above, the same procedure as in ExampleA4 was used to fabricate each of the solid electrolytic capacitors ofExample A9 to A18.

In the resultant solid electrolytic capacitors of Examples A9 to A18, aweight ratio W (Wt %) of the second material based on a total amount ofthe first material consisting of the silver particles having the mediandiameter of 3 μm and the second material consisting of the silverparticles reduced from the Ag₂O particles having the median diameter of0.5 μm and acetylene black was almost the same as the aforesaid weightratio in mixing. Each weight ratio W was 0.25 wt % in Example A9, 0.5 wt% in Example A10, 1 wt % in Example A11, 2 wt % in Example A12, 3 wt %in Example A13, 10 wt % in Example A14, 20 wt % in Example A15, 40 wt %in Example A16, 45 wt % in Example A17, 50 wt % in Example A18 and 5 wt% in Example A4.

In the same manner as above, the resultant solid electrolytic capacitorsof Examples A9 to A18 were each determined for the equivalent seriesresistance (ESR) at the frequency of 100 kHz. Then, an index number ofthe equivalent series resistance (ESR) of each of the solid electrolyticcapacitors of Examples A9 to A18 was determined on the basis of theequivalent series resistance (ESR) of the solid electrolytic capacitorof Example A4 defined as 100. The results are listed in Table 4 asbelow. TABLE 4 WEIGHT RATIO 3 μm 0.5 μm SILVER SILVER OXIDE(I) ACETYLENEW PARTICLES Ag₂O PARTICLES BLACK (wt %) ESR EXAMPLE A9 99.75 0.125 0.1250.25 145 EXAMPLE A10 99.5 0.25 0.25 0.5 115 EXAMPLE A11 99 0.5 0.5 1 105EXAMPLE A12 98 1 1 2 105 EXAMPLE A13 97 1 2 3 100 EXAMPLE A4 95 2.5 2.55 100 EXAMPLE A14 90 5 5 10 102 EXAMPLE A15 80 15 5 20 102 EXAMPLE A1660 35 5 40 103 EXAMPLE A17 55 40 5 45 135 EXAMPLE A18 50 45 5 50 140

As is apparent from the table, each of the solid electrolytic capacitorsof Examples A4 and A10 to A16 wherein the weight ratio W of the secondmaterial based on the total amount of the first material consisting ofthe silver particles having the median diameter of 3 μm and the secondmaterial consisting of the silver particles having the median diameterof 0.5 μm and acetylene black was set in a range of 0.5 to 40 wt %presented a notably decreased ESR as compared with the solidelectrolytic capacitor of Example A9 wherein the weight ratio W was 0.25wt % and the solid electrolytic capacitors of Examples A17 and A18wherein the weight ratio W was more than 40 wt %. In particular, the ESRin each of the solid electrolytic capacitors of Example A4 and ExamplesA13 to A16 wherein the weight ratio W was in a range of 3 to 40 wt % wasfurther decreased.

Examples A19, A20 and Comparative Example 5

The same procedure as in Example A1 was used to fabricate each of solidelectrolytic capacitors of Examples A19, A20 and Comparative Example 5,except that a drying temperature for drying the paste for the mixedlayer was changed. Each drying temperature was set to 170° C. in ExampleA19, 180° C. in Example A20 and 150° C. in Comparative Example 5. In acase where the drying temperature for drying the paste for the mixedlayer was set to 150° C. as in Comparative Example 5, the Ag₂O particlesin the paste for the mixed layer were not sufficiently reduced to thesilver particles.

In the same manner as above, the resultant solid electrolytic capacitorsof Examples A19, A20 and Comparative Example 5 were each determined forthe equivalent series resistance (ESR) at the frequency of 100 kHz.Then, an index number of the equivalent series resistance (ESR) of eachof the solid electrolytic capacitors of Examples A19, A20 andComparative Example 5 was determined on the basis of the equivalentseries resistance (ESR) of the solid electrolytic capacitor of ExampleA1 defined as 100. The results are listed in Table 5 as below. TABLE 5DRYING TEMPERATURE (° C.) ESR COMPARATIVE EXAMPLE 5 150 150 EXAMPLE A1160 100 EXAMPLE A19 170 100 EXAMPLE A20 180 100

As is apparent from the table, each of the solid electrolytic capacitorsof Examples A1, A19 and A20 wherein the drying temperature for dryingthe paste for the mixed layer was set to not less than 160° C. to reducethe Ag₂O particles sufficiently to the silver particles presented anotably decreased ESR as compared with the solid electrolytic capacitorof Comparative Example 5 wherein the drying temperature was set to 150°C. and the Ag₂O particles in the mixed layer were not sufficientlyreduced to silver particles.

Examples A1a, A1b and A1c and Comparative Examples 1a, 1b and 1c

The same procedure as in Example A1 was used to fabricate each of solidelectrolytic capacitors of Examples A1a, A1b, and A1c, except that atype of the binding agent for preparing the paste for the mixed layerwas changed.

Example A1a used an epoxy resin as the binding agent and diethyleneglycol monobutyl ether as the solvent. Example A1b used a polyesterresin as the binding agent and cyclohexanone as the solvent. Example A1cused a phenolic resin as the binding agent and propylene glycol as thesolvent. Except for the above, the same procedure as in Example A1 wasused to fabricate each of the solid electrolytic capacitors of ExamplesA1a, A1b and A1c.

The same procedure as in Comparative Example 1 was used to fabricateeach of solid electrolytic capacitors of Comparative Examples 1a, 1b,and 1c, except that a type of the binding agent for preparing the pastefor the mixed layer was changed.

Comparative Example 1a used the epoxy resin as the binding agent anddiethylene glycol monobutyl ether as the solvent. Comparative Example 1bused the polyester resin as the binding agent and cyclohexanone as thesolvent. Comparative Example 1c used the phenolic resin as the bindingagent and propylene glycol as the solvent. Except for the above, thesame procedure as in Comparative Example 1 was used to fabricate each ofthe solid electrolytic capacitors of Comparative Examples 1a, 1b and 1c.

In the same manner as above, the resultant solid electrolytic capacitorsof Examples A1a, A1b and A1c and Comparative Examples 1a, 1b and 1c wereeach determined for the equivalent series resistance (ESR) at thefrequency of 100 kHz. Then, an index number of the equivalent seriesresistance (ESR) of each of the solid electrolytic capacitors ofExamples A1a, A1b and A1c and Comparative Examples 1a, 1b and 1c wasdetermined on the basis of the equivalent series resistance (ESR) of thesolid electrolytic capacitor of Example A1 defined as 100. The resultsare listed in Table 6 as below. TABLE 6 TYPE OF BINDING AGENT ESREXAMPLE A1 polyamideimide 100 EXAMPLE A1a epoxy resin 105 EXAMPLE A1bpolyester resin 105 EXAMPLE A1c phenolic resin 160 COMPARATIVE EXAMPLE 1polyamideimide 160 COMPARATIVE EXAMPLE 1a epoxy resin 165 COMPARATIVEEXAMPLE 1b polyester resin 165 COMPARATIVE EXAMPLE 1b phenolic resin 200

As is apparent from the table, even in a case where the type of thebiding agent was changed, each of the solid electrolytic capacitors ofExamples A1a, A1b and A1c presented a notably decreased ESR as comparedwith the solid electrolytic capacitors of Comparative Examples 1a, 1band 1c.

In a comparison of values of the ESR according to the type of the bidingagent, in the case of using polyamideimiede(polyimide resin), epoxyresin or polyester resin, the ESR was notably decreased as compared withthe case of using phenolic resin.

Examples C1 to C3

The same procedure as in Example A4 was used to fabricate each of solidelectrolytic capacitors of Examples C1 to C3, except that the firstmaterial consisting of the silver particles of nearly spherical formhaving the median diameter of 3 μm and the second material consisting ofthe silver oxide particles of nearly spherical form having the mediandiameter of 0.5 μm contained in the mixed layer were changed.

In Example C1, scale-shaped silver particles wherein the median in themaximum length L was 3 μm and the ratio of the maximum length L to thethickness d (L/d) was 20 were used as the first material andscale-shaped silver particles wherein the median in the maximum lengthwas 0.5 [2 m and the ratio of the maximum length L to the thickness d(L/d) was 20 were used as the second material.

In Example C2, the scale-shaped silver particles wherein the median inthe maximum length L was 3 μm and the ratio of the maximum length L tothe thickness d (L/d) was 20 were used as the first material and thesilver particles of nearly spherical form having the median diameter of0.5 μm used in Example A4 were used as the second material.

In Example C3, the silver particles of nearly spherical form having themedian diameter of 3 μm used in Example A4 were used as the firstmaterial and scale-shaped silver particles wherein the median in themaximum length L was 0.5 μm and the ratio of the maximum length L to thethickness d (L/d) was 20 were used as the second material.

In the same manner as above, the resultant solid electrolytic capacitorsof Examples C1 to C3 were each determined for the equivalent seriesresistance (ESR) at the frequency of 100 kHz. Then, an index number ofthe equivalent series resistance (ESR) of each of the solid electrolyticcapacitors of Examples C1 to C3 was determined on the basis of theequivalent series resistance (ESR) of the solid electrolytic capacitorof Example A4 defined as 100. The results are listed in Table 7 asbelow. TABLE 7 SILVER PARTICLES SILVER PARTICLES OF FIRST OF SECONDMATERIAL MATERIAL ESR EXAMPLE C1 scale-shaped scale-shaped 65 EXAMPLE C2scale-shaped nearly spherical 93 form EXAMPLE C3 nearly sphericalscale-shaped 95 form EXAMPLE A4 nearly spherical nearly spherical 100form form

As is apparent from the table, each of the solid electrolytic capacitorsof Examples C1 to C3 using the scale-shaped silver particles at leasteither in the first material or in the second material presented anotably decreased ESR as compared with the solid electrolytic capacitorof Examples A4 using the silver particles of nearly spherical form inboth of the first material and the second material. In particular, theESR in the solid electrolytic capacitor of Example C1 using thescale-shaped silver particles in both of the first material and thesecond material was further decreased.

Examples C4 to C7

In Examples C4 to C7, the scale-shaped silver particles wherein themedian in the maximum length L was 3 μm were used as the first materialand the scale-shaped silver particles wherein the median in the maximumlength L was 0.5 μm were used in the same manner as Example C1. However,the ratio of the maximum length L to the thickness d (L/d) in thescale-shaped silver particles of the first material and the secondmaterial was changed. Except for the above, the same procedure as inExample C1 was used to fabricate each of solid electrolytic capacitorsof Examples C4 to C7.

In Example C4, scale-shaped silver particles wherein the ratio of themaximum length L to the thickness d (L/d) was 4 were used in the firstmaterial and the second material. In Example C5, scale-shaped silverparticles wherein the ratio of the maximum length L to the thickness d(L/d) was 50 in the first material and the second material were used. InExample C6, scale-shaped silver particles wherein the ratio of themaximum length L to the thickness d (L/d) was 100 were used in the firstmaterial and the second material. In Example C7, scale-shaped silverparticles wherein the ratio of the maximum length L to the thickness d(L/d) was 120 were used in the first material and the second material.

In the same manner as above, the resultant solid electrolytic capacitorsof Examples C4 to C7 were each determined for the equivalent seriesresistance (ESR) at the frequency of 100 kHz. Then, an index number ofthe equivalent series resistance (ESR) of each of the solid electrolyticcapacitors of Examples C4 to C7 was determined on the basis of theequivalent series resistance (ESR) of the solid electrolytic capacitorof Example A4 defined as 100. The results are listed in Table 8 asbelow. TABLE 8 SILVER SILVER PARTICLES OF PARTICLES OF FIRST MATERIALSECOND MATERIAL SHAPE L/d SHAPE L/d ESR EXAMPLE C4 scale-shaped 4scale-shaped 4 70 EXAMPLE C1 scale-shaped 20 scale-shaped 20 65 EXAMPLEC5 scale-shaped 50 scale-shaped 50 72 EXAMPLE C6 scale-shaped 100scale-shaped 100 74 EXAMPLE C7 scale-shaped 120 scale-shaped 120 92EXAMPLE A4 nearly about 1 nearly about 1 100 spherical spherical formform

As is apparent from the table, each of the solid electrolytic capacitorsof Examples C1 and C4 to C6 using the scale-shaped silver particleswherein the ratio of the maximum length L to the thickness d (L/d) inthe first material and the second material was in the range of 4 to 100presented a notably decreased ESR.

INDUSTRIAL APPLICABILITY

According to the solid electrolytic capacitor of the invention asdescribed above, the mixed layer containing the first materialconsisting of the silver particles having the median diameter of notless than 2 μm, the second material consisting of the conducting carbonparticles and/or the silver particles having the median diameter of 1 μmor less and the biding agent is formed as the cathode on theelectrolytic layer, so that the particles of the second materialconsisting of the conducting carbon particles and/or the silverparticles having the median diameter of 1 μm or less enter the spacesbetween the particles of the first material consisting of the silverparticles having the median diameter of not less than 2 μm in the mixedlayer. As a result, the conductivity in the mixed layer is improved, theadhering property between the mixed layer and the electrolytic layer isenhanced decreasing the contact resistance thereof, and the equivalentseries resistance in the high frequency regions is notably decreased.

Further, in the solid electrolytic capacitor, in a case where the carbonlayer is formed between the electrolytic layer and the mixed layer, theadhering property between the mixed layer and the carbon layer isenhanced decreasing the contact resistance thereof, and the equivalentseries resistance in the high frequency regions is further decreased.

Furthermore, in the solid electrolytic capacitor, the scale-shapedsilver particles of which thickness to the length is very small are usedin the first material and the second material in the mixed layer, sothat the contact area between the silver particles of the first materialand the silver particles of the second material is enlarged, resultingin still further decrease in the equivalent series resistance in thehigh frequency regions.

1. A solid electrolytic capacitor comprising an anode formed of at leastone metal selected from tantalum, niobium, titanium and tungsten, and adielectric layer, an electrolytic layer and a cathode disposed on theanode, wherein the cathode comprises a mixed layer containing a firstmaterial consisting of silver particles having an average particlediameter (median diameter) of not less than 2 μm, a second materialconsisting of conducting carbon particles and/or silver particles havingan average particle diameter (median diameter) of 1 μm or less and abinding agent.
 2. The solid electrolytic capacitor as claimed in claim1, wherein the cathode comprises a carbon layer formed between theelectrolytic layer and the mixed layer.
 3. The solid electrolyticcapacitor as claimed in claim 1, wherein an amount of the secondmaterial based on a total amount of the first material and the secondmaterial is set in a range of 0.5 to 40 wt %.
 4. The solid electrolyticcapacitor as claimed in claim 3, wherein the amount of the secondmaterial based on the total amount of the first material and the secondmaterial is set in a range of 3 to 40 wt %.
 5. The solid electrolyticcapacitor as claimed in claim 1, wherein the silver particles having theaverage particle diameter (median diameter) of 1 μm or less are reducedfrom silver oxide particles having the average particle diameter (mediandiameter) of 1 μm or less.
 6. The solid electrolytic capacitor asclaimed in claim 1, wherein the binding agent is at least one resinselected from polyimide resin, epoxy resin and polyester resin.
 7. Thesolid electrolytic capacitor as claimed in claim 1, wherein theconducting carbon particles are carbon black and/or graphite.
 8. Afabrication method for solid electrolytic capacitor of claim 1comprising a step of forming the mixed layer containing the silverparticles having the average particle diameter (median diameter) of 1 μmor less wherein the silver oxide particles having the average particlediameter (median diameter) of 1 μm or less contained in the mixed layerare reduced.
 9. The fabrication method for solid electrolytic capacitoras claimed in claim 8, wherein the silver oxide particles are reduced byheat-treatment at not less than 160° C.
 10. A solid electrolyticcapacitor comprising an anode formed of at least one metal selected fromtantalum, niobium, titanium and tungsten, and a dielectric layer, anelectrolytic layer and a cathode disposed on the anode, wherein thecathode comprises a mixed layer containing a first material consistingof scale-shaped silver particles having a median in a maximum length ofnot less than 2 μm and a second material consisting of conducting carbonparticles and/or silver particles having an average particle diameter(median diameter) of 1 μm or less and a binding agent.
 11. The solidelectrolytic capacitor as claimed in claim 10, wherein the cathodecomprises a carbon layer formed between the electrolytic layer and themixed layer.
 12. The solid electrolytic capacitor as claimed in claim10, wherein a ratio of a maximum length L to a thickness d (L/d) of thescale-shaped silver particles is set in a range of 4 to
 100. 13. Thesolid electrolytic capacitor as claimed in claim 10, wherein an amountof the second material based on a total amount of the first material andthe second material is set in a range of 0.5 to 40 wt %.
 14. The solidelectrolytic capacitor as claimed in claim 13, wherein the amount of thesecond material based on the total amount of the first material and thesecond material is set in a range of 3 to 40 wt %.
 15. A solidelectrolytic capacitor comprising an anode formed of at least one metalselected from tantalum, niobium, titanium and tungsten, and a dielectriclayer, an electrolytic layer and a cathode disposed on the anode,wherein the cathode comprises a mixed layer containing a first materialconsisting of silver particles having an average particle diameter(median diameter) of not less than 2 μm, a second material consisting ofconducting carbon particles and/or scale-shaped silver particles havinga median in a maximum length of 1 μm or less and a binding agent. 16.The solid electrolytic capacitor as claimed in claim 15, wherein thecathode comprises a carbon layer formed between the electrolytic layerand the mixed layer.
 17. The solid electrolytic capacitor as claimed inclaim 15, wherein a ratio of a maximum length L to a thickness d (L/d)of the scale-shaped silver particles is set in a range of 4 to
 100. 18.The solid electrolytic capacitor as claimed in claim 15, wherein anamount of the second material based on a total amount of the firstmaterial and the second material is set in a range of 0.5 to 40 wt %.19. The solid electrolytic capacitor as claimed in claim 18, wherein theamount of the second material based on the total amount of the firstmaterial and the second material is set in a range of 3 to 40 wt %. 20.A solid electrolytic capacitor comprising an anode formed of at leastone metal selected from tantalum, niobium, titanium and tungsten, and adielectric layer, an electrolytic layer and a cathode disposed on theanode, wherein the cathode comprises a mixed layer containing a firstmaterial consisting of scale-shaped silver particles having a median ina maximum length of not less than 2 μm, a second material consisting ofconducting carbon particles and/or scale-shaped silver particles havinga median in a maximum length of 1 μm or less and a binding agent. 21.The solid electrolytic capacitor as claimed in claim 20, wherein thecathode comprises a carbon layer formed between the electrolytic layerand the mixed layer.
 22. The solid electrolytic capacitor as claimed inclaim 20, wherein a ratio of a maximum length L to a thickness d (L/d)of the scale-shaped silver particles is set in a range of 4 to
 100. 23.The solid electrolytic capacitor as claimed in claim 20, wherein anamount of the second material based on a total amount of the firstmaterial and the second material is set in a range of 0.5 to 40 wt %.24. The solid electrolytic capacitor as claimed in claim 23, wherein theamount of the second material based on the total amount of the firstmaterial and the second material is set in a range of 3 to 40 wt %.